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Chapter 4 Clinical Radiation Generators

4.1 Kilovoltage Units

Machine Energy Treatment depth (90% depth dose)

Grenz-Ray therapy < 20 kV -

Contact therapy (endocavitary)

40-50 kV 1–2 mm

Superficial therapy 50-150 kV 5 mm

Orthovoltage therapy (deep therapy)

150-500 kV 2-3 cm

Supervoltage therapy 500-1000 kV

Megavoltage therapy > 1 MV

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4.1 Kilovoltage Units (cont’d)

• Filters (typically made of aluminum or copper) are often used to ‘harden’ the energy.

• Beam quality expressed as the half-value layer (HVL), defined as the thickness of a specified material that would reduce the exposure rate by half.

• High skin dose and sharp decrease of depth dose distributions are the major limitations of the low-energy machines.

• Conventional transformer is used for up to 300 kV. Resonant transformer units used for higher energies.

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Grenz rays HVL=0.04mm Al

Contact therapy HVL=1.5mm Al

Superficial therapy HVL=3.0mm Al

Orthovoltage HVL=2.0mm Cu

Cobalt-60

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4.2 Van de Graaff Generator

Fig. 4.4

An electrostatic accelerator designed to accelerate charged particles.

In radiotherapy, it accelerates electrons to produce high-energy x-rays, typically at 2 MV.

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4.3 Linear Accelerator

The linac uses high-frequency electromagnetic waves (3000 MHz microwaves, wavelength about 10 cm) to accelerate electrons to high energies through a linear tube.

There are two types of design used in radiotherapy: traveling and stationary (standing) waves.

The stationary wave design provides maximum reflection on both ends, when combined with the forward wave, forms the stationary wave.

The traveling wave design requires a terminating load at the end of the structure to prevent reflected wave.

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4.3 Linear Accelerator (cont’d)

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4.3 Linear Accelerator (magnetron & klystron)

The magnetron is a device that produces microwave pulses of several microseconds duration at a repetition rate of several hundreds pulses per second. Typically, magnetron is used for low-energy linacs (6 MV or less).

The klystron is a microwave amplifier that needs to be driven by a low-power microwave oscillator. It is used for higher energy linacs.

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4.3 Linear Accelerator (X-ray beam)

Bremsstrahlung x-rays are produced when the electrons are incident on a target of a high-Z material (tungsten).

The energy spectrum is continuous with the maximum energy equal to the incident electron energy. The average energy is about 1/3 of the maximum energy.

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4.3 Linear Accelerator (target and flattening filter)

The electrons hitting the target are in the MeV range, as a result, the x-rays produced from the target are forward peaked. In order to make the intensity more uniform, a flattening filter is inserted in the beam before it leaves the treatment head.

The flattening filter is usually made of lead, although tungsten, uranium, steel, aluminum have also been used. The geometrical shape of the flattening filter is usually designed to produce a flat field at 10 cm depth in water.

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4.3 Linear Accelerator (beam collimation and monitoring)

The treatment beam is first collimated by a fixed primary collimator.

Below that is the monitor chamber, which monitors the dose rate, integrated dose, and field symmetry. The monitor chamber is sealed, so that its response is NOT affected by the temperature and pressure in the treatment room.

After passing through the monitor chamber, the beam is further collimated by movable x-ray collimators (jaws), which provide a range of field sizes from

00 up to 4040 cm2.

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4.3 Linear Accelerator (electron beam)

The electron beam, as it emerges from the waveguide tube, has a diameter of about 2-3 mm. In the electron mode of linac operation, this beam is made to strike an electron scattering foil in order to spread the beam to wider area as well as to get a more uniform fluence across the treatment field. The scattering foil consists of thin metallic foil (Pb), so that most electrons are scattered. But some bremsstrahlung photons are also produced, called x-ray contamination of the electron beam.

In some linacs, the broadening of the electron beam is accomplished by electromagnetic scanning.

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For electron beams, applicators (cones) are used which are close to the treatment surface to minimize the effects of in-air scattering.

Note: Jaw setting are fixed for each particular energy and applicator size.

4.3 Linear Accelerator (electron beam)

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4.3 Linear Accelerator (gantry)

Modern-day linacs are constructed so that the source of radiation can rotate about a horizontal axis. The intersection of this axis and the collimator axis (beam central axis) is the isocenter.

The source-to-isocenter distance for most machines is 100 cm.

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4.4 Betatron (medical use by JS Laughlin 1950s)

An electron in a changing magnetic field experiences acceleration in a circular orbit.

Betatron can produce a wide range of energies from less than 6 to more than 40 MeV. But compared to linacs, betatron has lower dose rate and smaller field size.

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4.5 Microtron

The microtron is a combination of a linac and a cyclotron. The electrons attain higher energies by repeated passing through the linac component.

The final treatment beam can be extracted at different orbit positions for different desired energies.

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4.6 Cyclotron (developed by E.O.Lawrence in the 1930s at UC Berkeley)

Charged particles are accelerated as they leave one D and enter another D. The particles travel in circular orbits due to the magnetic field. With increased energy, the radius of the orbit also increases.

Cyclotrons are used for proton and neutron machines

Alternating potential is applied to the 2 Ds.

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4.7 Machines Using Radionuclides

radionuclide half-life (years)

-ray energy (MeV)

Value (Rm2/Ci-h)

Specific activity (Ci/g)

Ra-226

(0.5 mm Pt)

1622 0.83 0.825 ~0.98

Cs-137 30 0.66 0.326 ~50

Co-60 5.26 1.25

(1.17, 1.33)

1.30 ~200

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4.7 Machines Using Radionuclides (Co-60 unit)

The Co-60 is produced in a nuclear reactor from Co-59 by 59Co(n,)60Co

Co6027 (5.26 y)

-(Emax=0.32 MeV), 99+%

-(Emax=1.48 MeV),

0.1%

Ni6028

2.50

1.33

(1.33 MeV)

(1.17 MeV)

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4.7 Machines Using Radionuclides (Co-60 unit, cont’d)

A typical 60Co source is a cylindrical disc of diameter ranging from 1.0 to 2.0 cm, which gives rise to geometric penumbra.

Due to interaction of the primary –ray and the source itself and other surrounding materials in the treatment head, there are low-energy contaminants which contribute to about 10% of the total intensity.

In addition, contaminant electrons are also produced due to these interactions.

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4.7 Machines Using Radionuclides (Co-60 unit, cont’d)

The source is contained inside a stainless-steel capsule and sealed by welding. This capsule itself is again contained in another stainless-steel capsule sealed by welding, for radiation safety reasons.

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4.7 Machines Using Radionuclides (Co-60 unit, cont’d)

The source head consists of a steel shell filled with lead and a device (typically a pneumatic device) for bringing the source in front of an opening when the beam is ‘on’.

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4.7 Machines Using Radionuclides (Co-60 unit, cont’d)

geometric penumbra:

SDD

SDDdSSDsPd

)(

To reduce the geometric penumbra, trimmers can be used to increase SDD.

d

s

Pd

SDD

SSD

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4.8 Heavy Particle Beams

These particles include neutrons, protons, deuterons, - particles, negative pions, and heavy ions.

They have the advantages in terms of dose localization and therapeutic gains (greater dose on tumor than on normal tissues).

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4.8 Heavy Particle Beams (neutrons)

High energy neutron beams may be produced by D-T generators, cyclotrons, linacs, and nuclear reactors.

MeVnHeHH 6.1710

42

31

21 D-T generator:

nBBeH 10

105

94

21 cyclotron:

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4.8 Heavy Particle Beams (protons, 150-250 MeV)

The major advantage of high energy protons and other heavy charged particles is their characteristic ‘Bragg peak’ in the depth dose distribution.

Particles with the same MeV/A have approximately the same velocities and range. For example, 150 MeV protons, 300 MeV deuterons and 600 MeV helium ions have approx. the same range in water 16 cm.

0

50

100

dose

Depth in water (cm)

0 4 8 12 16 20

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4.8 Heavy Particle Beams (negative pions)

Negative pions () of energy close to 100 MeV have been used in radiotherapy, providing a range of 24 cm in water.

The Bragg peak exhibited by pions is more pronounced than other heavy charged particles because of the additional nuclear disintegration by pion capture, called star formation.